A telephone handset typically comprises an elongate housing which defines a cavity in which a first converter such as a microphone is arranged at one end for converting received acoustic signals into an electrical signal and wherein a second converter such as a loudspeaker or telephone is arranged at another end for converting received electrical signals into an audible acoustic signal. For the sake of simplicity the respective terms microphone and loudspeaker are used in the following, without the intention of limitation, to designate the said converters.
In a telephone handset of the type for use in combination with a telephone apparatus, in general only connecting wires extend from the one to the other end in the said cavity. In a telephone handset of the integrated type the electrical and electronic components necessary for functioning are also accommodated in said cavity. In the English-language professional literature both types are called a "handset".
Particularly the above mentioned portable, cordless telephone handsets with built-in transmitting and receiving facilities have in the course of time become more compact and convenient to handle in terms of size as a result of a far-reaching miniaturization and/or integration of the electrical and electronic components. The distance between the microphone and the loudspeaker has hereby become smaller, wherein comparatively more sensitive microphones are also employed for adequate reception of speech sound etc.
This has the drawback however that the microphone also more easily picks up noise and vibrations generated by the loudspeaker. As a consequence of delays in the speech transfer the person being spoken to at the remote end during a telephone conversation will hereby have relayed back to him his own delayed voice sound, which manifests itself as an intrusive echo.
It has been found that an echo of the same level becomes the more disturbing as the delay time increases. This problem will for instance occur in transmission paths running via communication satellites, but in particular also in so-called TDMA/TDD (Time Division Multiple Access/Time Division Duplex) communication systems, for instance digital cordless communications equipment operating in accordance with the DECT standard (Digital European Cordless Telecommunications).
Use is made herein of a number of speech channels arranged in time sequence (TDMA). A first half of this number of channels is used for communication in the one direction while a second half is used for communication in the other direction (TDD). This causes an additional delay compared for instance with systems having communication channels continuously available for each direction. Particularly in the case of telephone handsets for use in such systems this additional delay imposes requirements as to the maximum permissible sound and vibration transfer or cross-talk between the loudspeaker and the microphone.
Different cross-talk paths between the microphone and the loudspeaker of a telephone handset can be distinguished: an external acoustic path through the air outside the telephone handset, an internal acoustic path via the cavity in the housing of the telephone handset and a number of mechanical vibration paths, wherein the vibrations generated by the loudspeaker are transferred mechanically via the housing, via the electrical and electronic components, via the wiring etc. to the microphone. Cross-talk can also occur electrically.
In the English-language professional literature the above described forms of cross-talk are known under the concept of "Telephone Coupling Loss (TCL)". Individual TCL values, laid down in standard specifications, apply for the different transmission systems.
From DE-A1-35 16 290 is known a telephone handset of the type for use with a telephone apparatus. The grip of the handset is provided with a cover which damps mechanical vibrations. The mechanical vibration decoupling between the loudspeaker and the microphone is however far from optimal because the shells encasing the microphone and the loudspeaker are coupled via mechanical snap-in connections to a mechanically rigid carrier and the microphone and the loudspeaker make direct mechanical contact with the shells. This telephone receiver does not have an internal connecting cavity between the microphone and the loudspeaker.
EP-A-0,166,198 likewise relates to a telephone handset for use in combination with a telephone apparatus. The loudspeaker and microphone are mounted in the housing via elastic sealing rings in order to prevent acoustic cross-talk via the cavity of the housing. In order to hold the microphone and loudspeaker firmly in position the housing is provided with various resilient fingers and cams. Steps which make no contribution whatever toward reducing the mechanical vibration transfer between loudspeaker and microphone.
EP-A-0,453,061 relates to a telephone handset of the integrated type wherein the microphone, loudspeaker and the several electrical and electronic components are arranged in a cavity inside a single housing. In order to improve the mechanical decoupling the microphone is mounted in a cylindrical chamber of the housing via a so-called "acoustic insulator". This acoustic insulator consists fundamentally of a compressible gasket with a first portion provided with a larger opening for receiving the microphone and a narrower second portion which extends mutually in line with the first portion and can protrude outside the housing of the telephone handset.
The insulator has to engage in firmly clamping manner on the microphone and the wall of the chamber to prevent the insulator and the microphone coming out of the chamber as a result of shocks or impacts. This requirement imposes limitations on the choice of material for the insulator. Furthermore, the insulator is hereby subjected continually to a considerable mechanical stress, which in the long term has an adverse effect on the clamping action of the material. The danger of undesired movement of the microphone in relation to the chamber is hereby increased, as is the danger of the occurrence of undesired acoustic leakage paths via the cavity in the housing.
GB-A-2,225,192 discloses a telephone handset of the type for use with a telephone apparatus, wherein the microphone and loudspeaker are likewise mounted in firmly clamping manner in respective chambers of the housing by means of elastic gaskets. The drawbacks of a limited material choice and a continuous, comparatively high mechanical stress in the gaskets as discussed above also apply here.